Conventionally, as a factor for limiting increases in speed and distance of an optical fiber communication system, polarization mode dispersion (PMD) is known.
PMD is a phenomenon in which birefringence of an optical fiber causes a differencial group delay between two orthogonal polarization modes propagating inside the optical fiber. Birefringence is a phenomenon in which a noncircular core of an optical fiber or an anisotropic stress such as an external pressure applied to the optical fiber lifts degeneracy between the two orthogonal polarization mode components propagating inside the optical fiber, and a difference is generated between the refractive indexes of the respective modes. Birefringence (B) as a parameter indicating the level of birefringence is expressed as the following expression (1).B=|nx−ny|  (1)Herein, nx and ny denote effective indexes of orthogonal polarization modes respectively.
When an optical fiber has uniform birefringence in a certain section, propagating light propagates while changing its polarization state periodically. This period is called beat length (LB), and has a relationship with birefringence (B) expressed as the following expression (2).LB=λ/B  (2)Herein, λ denotes a light wavelength.
A noncircular core in the above-described optical fiber and a stress on an optical fiber are not uniform in the longitudinal direction, so that a technique for measuring polarization characteristic distributions of birefringence and beat length, etc., in the longitudinal direction of an optical fiber has been demanded.
To satisfy this demand, as a technique for measuring polarization characteristic distributions along the longitudinal direction of the optical fiber, Patent Document 1 discloses a POTDR (Polarization Optical Time Domain Reflectometer). However, the distance resolution in the POTDR is generally in a meter order, and this is not suitable for measuring a short beat length and fiber characteristic changes in a short section.
Patent Document 2 discloses a beat length measuring technique using Brillouin scattering. The technique disclosed in Patent Document 2 adopts BOFDA (Brillouin Optical Frequency Domain Analysis) as a method of measuring a Brillouin gain, and its distance resolution is 1.22 meters and 5.5 meters. However, even by using the BOFDA, it is difficult to measure optical fiber characteristics such as oscillation of Brillouin gain, that is, beat length relating to PMD with a higher distance resolution.
On the other hand, Patent Document 3 discloses Brillouin Optical Correlation Domain Analysis (BOCDA) as a Brillouin gain measuring method using Brillouin scattering.
Brillouin scattering is a phenomenon in which, when light (pumping light) propagates inside an optical fiber, the pumping light generates an acoustic wave in the optical fiber, and due to interaction of the pumping light and the acoustic wave, a part of the pumping light is scattered to the rear side. In this case, the frequency of the scattered light is shifted to the lower-frequency side. When light (probe light) which propagates opposite to pumping light is propagated, scattered light generated inside the optical fiber becomes a gain to amplify the probe light. In a silica glass optical fiber, the gain becomes maximum when the frequency difference between the pumping light and the probe light is about 10 GHz, and the gain given to the probe light at this time is called Brillouin gain.
The Brillouin gain is also changed by a relative polarization state of the pumping light and the probe light. For example, at a certain position inside an optical fiber, when polarizations of the pumping light and the probe light coincide with each other, the Brillouin gain becomes maximum, and when the polarizations are orthogonal, the Brillouin gain becomes zero.
When an optical fiber has uniform birefringence in a certain section, the pumping light and the probe light propagate opposite to each other while changing their polarization states periodically by setting the beat length (LB) as one period. From this fact, the Brillouin gain also fluctuates periodically along the longitudinal direction of the optical fiber, and its fluctuation periods (LP) are expressed as the following expression (3) (see FIG. 1).LP=LB/2  (3)
FIG. 1 is a view for describing the principle of beat length measurement using Brillouin scattering. In FIG. 1, the area (a) shows measurement positions P1 to P4 of an optical fiber 7 as a measuring object. The area (b) shows pumping light polarization states at the respective measurement positions P1 to P4 of the optical fiber 7 shown in the area (a). The area (c) shows probe light polarization states at the respective measurement positions P1 to P4 of the optical fiber 7 shown in the area (a). The area (d) shows a Brillouin gain along the longitudinal direction of the optical fiber 7 and periods thereof (Brillouin periods).    Patent Document 1: U.S. Pat. No. 6,724,469    Patent Document 2: T. Gogolla et al., “Distributed Beat Length Measurement in Single-Mode Optical Fibers Using Stimulated Brillouin-Scattering and Frequency-Domain Analysis,” Journal of Lightwave Tech., Vol. 18, No. 3, pp. 320-328 (March 2000)    Patent Document 3: Japanese Patent No. 3667132    Non-patent Document 1: Press, et al., “Numerical Recipes in C, Second Edition,” Chapter 12, Cambridge University Press.